1. Field of Invention
The present invention relates generally to an apparatus and concomitant method for monitoring processes in a semiconductor process chamber. More specifically, the invention relates to an apparatus that detects a plasma in the process chamber.
2. Background of Invention
During many semiconductor processing operations, it is critical to maintain a substrate stationary during processing. To achieve these ends, substrate support pedestals often are equipped with substrate retaining devices such as electrostatic, mechanical, and/or vacuum chucks in order to hold the substrate to a support surface of the pedestal. Pedestals equipped with electrostatic chucks are commonly chosen as retaining devices because of their rapid activation and deactivation times, low likelihood of substrate damage and exposure of the entire wafer face during processing.
Electrostatic chucks operate by supplying DC voltage to an embedded electrode within a dielectric material. The applied voltage produces a charge on the support surface of the electrostatic chuck, which in turn induces an electrostatic potential of opposite polarity on a backside of the substrate adjacent the support surface. This electrostatic potential affixes a substrate to the pedestal during processing.
The resistivity of the support surface is an important parameter for chucking and de-chucking performance (i.e., the repeated clamping and releasing of the substrate from the support surface). Maintaining an intended characteristic resistivity at the support surface prevents charge migration and current flow that degrades the chucking force. Contaminants upon the chuck surface often increase the resistively of the chuck thereby parasitically altering the chucking performance. As such, the support surface of the electrostatic chuck must be free of contaminants in order to function reliably. Once substantial current leakage occurs or a residual charge is established within the electrostatic chuck, the result is a reduced or total loss of chucking force.
A common form of electrostatic chuck surface contamination is the absorption of gases, or their reaction with the support surface of the electrostatic chuck (typically a ceramic material), when the process chamber is vented to atmosphere. The exposure of the support surface containing residual atmospheric gases to high temperatures during wafer processing creates a low resistance contamination film across the support surface. Over time, the repeated venting of the chamber and exposure to elevated temperatures during processing multiple wafers cases, the contamination film increase in thickness and decreases in resistance. When the resistivity of the contamination film is lower than that of the ceramic, the electrostatic chuck begins to set up the opposite polarity charge in the contamination film itself, and not the substrate on top of the contamination film. Thus, chucking force between the substrate and the pedestal is lost.
The impact of the contamination film on chucking performance depends on the thickness and resistivity of the contamination film as well as the operating temperature of the electrostatic chuck. Because the bulk resistivity of the electrostatic chuck material is inversely proportional to its temperature, the impact of the conductive contamination film is more severe at lower temperatures where the bulk resistivity is higher. Hence, if weak chucking force is observed at higher temperatures, the electrostatic chuck will exhibit almost no chucking force at lower temperatures. The primary variables which govern the formation of a contamination film on the chuck are operating temperature, time at temperature, and time of exposure to atmosphere.
The contamination film will continue to grow on the surface of the electrostatic chuck until the formed contamination film is removed by a maintenance procedure. Maintenance is performed periodically to remove contaminant films from the electrostatic chuck support surface.
One maintenance procedure consists of a low power in-situ plasma etch which sputters contaminants off the support surface of the electrostatic chuck. To perform this maintenance procedure, an RF generator, an auto-tuning RF match, and a service controller are installed on the applicable chamber. A plasma is generated within process chamber by applying RF power to the electrodes within the electrostatic chuck, while flowing argon gas into the chamber. Negative bias on the chuck, with respect to the plasma, causes argon ion bombardment of the chuck surface, wherein the ions xe2x80x9csputterxe2x80x9d off the contaminant layer. After the plasma etch has been performed and all contaminants have been removed, the electrostatic chuck has been restored to a condition to run substrates until the next maintenance service interval.
One problem associated with using low power plasma etching is the difficulty in confirming that the plasma has been struck, initiating the cleaning, or etch cycle. Some process chambers are equipped with windows that allow viewing of the interior of the chamber. Thus, a user may be able to visually identify the presence of the plasma by viewing the plasma xe2x80x9cglowxe2x80x9d. However, not all chambers have the window positioned to allow for easy viewing of the plasma, while other chambers are fitted with process kits that frequently obstruct the line of sight between the window and the portion of the chamber containing the plasma. As such, verification of the presence of the plasma is often very difficult.
If the removal of the contaminants from the electrostatic chuck is not successful, the cleaning process must be repeated. This repetition of the maintenance procedure leads to increased process chamber downtime, and correspondingly, reduced product throughput. Therefore, there is a need in the art for an apparatus that facilitates the detection of plasma in a semiconductor process chamber.
The disadvantages associated with the prior art are overcome by a plasma detection system that facilitates determining a presence of a plasma within a semiconductor process chamber. A plasma detection system comprises a floating contact, i.e., electrically xe2x80x9cfloatingxe2x80x9d from ground, exposed to a plasma forming region of a process chamber and coupled to a measuring device. The measuring device detects an increase in voltage on the floating contact when the plasma is struck in the plasma forming region, thus indicating the presence of the plasma in the process chamber.
A method for detecting presence of a plasma in a processing chamber is also disclosed. The method comprises the steps of electrically floating a contact exposed to a plasma forming region of the processing chamber, striking a plasma in the plasma forming region and measuring a voltage level of the contact.